This R21 grant application aims to understand the earliest events required to initiate gingival inflammation, with an ultimate goal of understanding the complete process of periodontitis development. Periodontal diseases especially periodontitis affect the majority of people 65 years and older, and is the major cause of tooth loss in this population. Development of periodontitis involves a serial transition from healthy gum to gingivitis (gum inflammation) to periodontitis (progressive periodontal tissue destruction and alveolar bone loss). It has been well established that periodontitis results from dysbiotic interaction between the subgingival microbiota and the host. F. nucleatum (Fn) has been found to be associated with gingivitis, while P. gingivalis (Pg) is considered a keystone pathogen in periodontitis as it has been shown to trigger dysfunctional host response as well as to influence community ecology of the subgingival microbiota. Although we have ample knowledge about the pathogenesis mechanisms of periodontitis, we know nothing about the earliest events happening in the supragingival plaque that set the stage for future development of disease. Based on findings from both in vitro and in vivo studies, we hypothesize that Veillonella (Va) serves as keystone species in early biofilm (supragingival plaque) development by allowing colonization and growth of periodontopathogens, and that by doing so skews biofilm ecology towards disease. A 3-species model is used, with F. nucleatum (Fn) representing gingivitis pathogen, Pg representing periodontal pathogen, and the transformable strain V. atypica OK5 representing Va. In Aim 1 we will test the hypothesis that Va produces vitamin K and heme, two essential nutrients for Fn and Pg, and that expression of the vitamin K and heme biosynthesis genes is induced or upregulated by coaggregation with Fn and Pg. In Aim 2 we will determine the mechanism of coaggregation between Va and Fn and Pg. Completion of these aims is made possible by our recent development of the first and only tractable transformation system in Veillonella.